The major objective of this proposal is to further investigate the important functions of the long cytoplasmic domains of the HIV-1 and SIV envelope glycoproteins. Particular emphasis will be placed on the role of the cytoplasmic domain in modulating the fusion activity of the envelope glycoprotein, and in intracellular transport and assembly of the envelope protein into virus particles. We have recently discovered that a truncation of the cytoplasmic tail of the SIV TM protein markedly enhances cells fusion activity, without detectably affecting the steady state level of expression of proteins on the cell surface. The mechanism by which this change in the intracellular part of the molecule can affect the biological activity of the external domain of the protein is of particular interest, and will be investigated with respect to possible alteration of receptor interaction, protein conformation, lateral mobility, and interaction with cytoskeletal components. A fluorescence dequenching assay will be developed to quantitatively determine the differences in fusion activity of glycoprotein constructs. To further investigate the role of specific sequences of the HIV and SIV glycoproteins in the induction of fusion of CD4+ target cells, we will investigate the effects of a series of progressive truncations or other modifications of the cytoplasmic domain on fusion activity. We will also analyze deletions and site-directed mutants to examine the possible role of two conserved tyrosine-containing regions in the cytoplasmic domain in gp4l in transport of the envelope protein and its stability on the cell surface. We will use expression systems to study the specificity of the interactions between envelope and core proteins in virus assembly, identify the precise protein domains in gp4l which are involved in the interaction between the envelope and the core proteins, and investigate the potential of peptide analogs of this domain to serve as specific inhibitors of virus assembly.